1. Field of the Invention
This invention relates to a crystallization process of L-.alpha.-aspartyl-L-phenylalanine methyl ester which is easy to handle in the filtration and dehydration operations.
2. Description of the Related Art
L-.alpha.-aspartyl-L-phenylalanine methyl ester (hereinafter referred to as to APM) is a substance expected to be used widely as a low-caloric sweetening agent owing to its excellent sweetness. A variety of processes have so far been known to produce APM industrially. Upon execution of any of these processes, however, a crystallization step is indispensable in order to isolate APM from the reaction solution to prepare it as a product.
Generally in the crystallization step, a crude product is dissolved again in water, an organic solvent or a hydrated organic solvent, and the resultant solution is allowed to cool through heat-exchange with a coolant or partial vaporization of the solvent under reduced pressure to deposit the crystals in a crystallization apparatus equipped with stirring blades, the crystals being then filtered or dehydrated by means of a centrifuge or the like. There is also known a process for preparing the crystals which comprises cooling an aqueous APM solution of a certain concentration or higher through conduction heat transfer without giving forced movement such as mechanical agitation to obtain an apparent ice-cream-like pseudo-solid and, if necessary, cooling further the pseudo-solid thus formed.
However, in the case of using crystallization apparatuses equipped with conventional stirrers, APM obtained, for instance, in a crystallization apparatus with a paddle- or turbine-type stirring means presents a fine needle-like crystal habit so that its solid-liquid separation in filtration or dehydration is very insufficient, thus raising serious problems upon practice.
Further, when the solid-liquid separation is repeated, the basal layer of the cake becomes hard by compression and its removal takes many hours to complete. Moreover, in a drying step subsequent to the crystallization step, the larger moisture content of the cake leads to a higher drying load, and the larger bulk specific volume of the resulting dry powder also causes its handling to be very difficult.
A process, in which cooling is effected only by conduction heat transfer without agitation of the system, takes a very long time to operate and hence can hardly be put into industrial practice.